Long-Range Electron Transfer Across Peptide Bridges:  The Transition from Electron Superexchange to Hopping

Long-range electron transfer rate constants for complexes of the type [(bpy)2RuIIL−Pro n −apyRuIII)(NH3)5]5++ proline residues (n) varying from 0 to 9 were determined by complementary electron pulse radiolysis and flash photolysis techniques from the picosecond to the millisecond time scales. The activationless k max values from both techniques coalesce into one data set. The distance dependence of the reactions is consistent with a smooth transition from a superexchange mechanism with attenuation constant β = 1.4 Å-1 to a hopping mechanism with attenuation constant β = 0.17 Å-1. The transition occurs between n = 3 and 4 prolines, and the virtual hopping rate constant at the shortest distance is about 1 x 106 times slower than that observed for the superexchange value.